A400M?
Simplistically put, but how about taking the electronic bits left over from the Nimrods (if they kept them that is) and make up palletised roll in/roll out mission modules that will fit in the back of a A400M and mount the sensors etc on wing hard points (if there any of course).
Or use the same airframe as the new supa-dooper tanker so you cut cost on the maintenance and flight deck training side of things?
Could always just buy a few 'Bears' (they gotta be well cheap these days) and then shoehorn the Nimrod bits in....